As a variable valve mechanism provided for an internal combustion engine and capable of changing the opening and closing timings and the maximum lift amount of an engine valve, there are, for example, those disclosed in U.S. Pat. No. 6,019,076 and U.S. Pat. No. 6,401,677 (B1).
The variable valve mechanism disclosed in U.S. Pat. No. 6,019,076 comprises a swinging cam swingably supported on a camshaft for the purpose of opening and closing an intake valve or an exhaust valve, a control shaft driven by an actuator, a control lever having a drive pin and mounted to the control shaft, a control member which has a rocker lever for swinging the swinging cam by being swung by a rotary cam rotated integrally with the camshaft and which converts the motion of the control lever into a swinging motion of the swinging cam, and a spiral spring for pressing the rocker lever against the rotary cam. The control lever and the control member are connected so as to be capable of relative motions through the engagement of a slider provided on the drive pin with a slot formed in a frame of the control member, and the spiral spring is provided between the swinging cam and the frame.
In addition, the variable valve mechanism disclosed in U.S. Pat. No. 6,401,677 (B1) comprises an output cam swingably supported on a camshaft for the purpose of opening and closing an intake valve or an exhaust valve, a frame which supports a rocker arm for swinging the output cam by being swung by an input cam rotated integrally with the camshaft and which is driven by an actuator to swing about the camshaft, and a coil spring for pressing the rocker arm against the input cam. The coil spring is curved in an arcuate shape in its natural state, one end portion of the coil spring is held by a spring receiving cup formed integrally with the output cam, and the other end of the coil spring is held by a spring receiving cup formed integrally with the frame.
In U.S. Pat. No. 6,019,076, the control shaft driven to rotate by the actuator swings the frame, which has a slot for engagement with a slider of the control lever, about the camshaft through the slider and thereby swings the swinging cam, whereby the opening and closing timings and the maximum lift amount of the engine valve are changed. Here, a slight gap, or play, is present between the slider and the frame, for securing smooth movements of both the members. The presence of the play, however, makes it difficult to accurately transmit a motion from the control lever to the frame. For controlling the valve operation characteristics with high accuracy, therefore, it is preferable to eliminate such a play.
Besides, in U.S. Pat. No. 6,019,076, the spring for pressing the rocker lever against the rotary cam is a spiral spring so shaped as to surround the camshaft in the circumferential direction of the camshaft over the entire circumference and a plurality of times. Therefore, the variable valve mechanism is large in size. On the other hand, in U.S. Pat. No. 6,401,677 (B1), the spring for pressing the rocker arm against the input cam is a coil spring disposed over a partial range in the circumferential direction of the camshaft, and, therefore, the spring is compact in form. However, the coil spring is a special spring which is curved in an arcuate shape, so that the coil spring is high in cost.
The present invention has been made in consideration of the above-mentioned circumstances. Accordingly, it is an object of the invention to eliminate the play at a connection portion in a valve characteristic varying mechanism and to enhance the accuracy in control of valve operation characteristics. Further, it is an object of the invention to maintain a highly accurate control of the valve operation characteristics, without being affected by the opening and closing operations of the engine valve, and to restrain the abrasion due to sliding at the connection portion. A further object of the invention is to enhance the accuracy in detection of operating conditions of the valve characteristic varying mechanism, for further enhancing the accuracy in control of the valve operation characteristics. It is also an object of the invention to simplify the structure of the valve characteristic varying mechanism, and to make it possible to use a straight hollow cylindrical coil spring as a control spring for the rocker arm, thereby to reduce the cost of the valve system, to securely hold the control spring and to enhance the durability of the control spring.